3.4 Storage

CO2 can be stored in several different types of geological formations, such as exhausted oil and gas fields, saline aquifers, basalts, and similar porous formations. The largest storage capacity is found in saline aquifers.8 A saline aquifer is a porous sandstone with salt water that is isolated from ground water and sea water. Of particular interest are formations of more than 800 meters depth, which are located under impermeable layers of cap rock. Under these conditions, CO2 is trapped in high density form. Just as oil and gas have remained for millions of years in porous formations due to such layers of rock, permanent storage of liquid CO2 can be sustained in the pores of the sandstone.

3.4.1 Previous experiences and ongoing storage projects

CO2 occurs in a concentrated form in natural reservoirs in the earth’s crust. Examples of such instances are the McElmo Dome in Colorado and the Bravo Dome in New Mexico. At these sites, CO2 has been stored for millions of years, similar to oil and gas deposits underground.41

Since the 1970s, CO2 has been used to increase the extraction from the oil fields in West Texas. The CO2 is pumped down in order to raise the pressure within the oil wells, and this allows for more oil to be extracted. This is called EOR, Enhanced Oil Recovery. The CO2 used for EOR is obtained mainly from natural underground CO2 deposits, meaning that there are currently no climate benefits stemming from these actions. It is for the purpose of EOR that the major pipelines to the oil fields in West Texas have been built, including the previously mentioned Cortez pipeline.

In the 1990s, the first CCS project with the explicit purpose of reducing CO2 emissions was initiated by the Norwegian oil company Statoil.

At their North Sea platform Sleipner, natural gas is extracted by Statoil. The gas is initially mixed with CO2, but in order to augment the value of the gas, some of the CO2 is later removed. This process is the so called “natural gas sweetening”. Since 1996 Statoil has injected 1 000 000 tonnes of CO2 each year into the formation named Utsira at a depth of 800 m, below the ocean floor. This exempts Statoil from carbon tax, as they would otherwise have had to pay taxes for every tonne of CO2 they emit. If the CCS operation didn’t take place, this would be among the ten largest emission points in Norway. So far, more than ten million tonnes have been injected through the same injection well with very good results.42

Figure 12 Drilling of storage well in North Dakota, US. Photo courtesy of Wes Peck at EERC/Univ. North Dakota

Presently, there are more than 155 integrated CCS projects in operation, under construction or in various stages of planning. These projects represent 176 million tonnes of annually stored CO2.43 In order to meet the emission targets set up by the IEA within the next decades, there is a need for thousands of plants, storing billion of tonnes annually.44 As shown in the projects section of this report, at this point very few of these initiatives and efforts are BECCS related, but are rather focusing on fossil fuels.

3.4.2 Storage security

During the nearly 40 years that CO2 has been stored in order to increase oil production through EOR, extensive experience with the technology has been gained. It can no longer be described as neither unproven nor unsafe, although the reasons for injecting CO2 have been to extract more fossil fuels, rather than to achieve climate benefits. Moreover, CO2 should not be compared with environmental wastes and toxins; it is not a toxic gas in lower concentrations and does not bring about permanent damage, even in cases of leakage or emissions. It is a naturally occurring gas that is deadly only in very high concentrations. Still, storage security is important both for local safety as well as for long-term climate change related reasons.

In the saline aquifer the lock-in of CO2 involves four successive processes. At first, injected CO2 is in a liquid phase. It is lighter than salt water and striving upwards in the storage formation. It is prevented to penetrate to the surface by a non-porous rock cover, the so called cap rock.

When CO2 disperses into the sandstone, it is also trapped in the stone’s pores, where it is prevented from migrating further. After ten years, more than a quarter of the liquid has become trapped in the pores in this manner. The third phase is slower and is composed of reactions in the saltwater, where CO2 is dissolved, making it heavier than water so that it begins striving downwards. After 100 years, about a quarter of the liquid CO2 is converted into a liquid that no longer strives upwards. The last phase is the slowest one and involves a reaction in which CO2 is converted into a so-called carbonate, a mineral that becomes part of the rock, and that will remain in this form for millions of years. As the process continues, there is a gradual decline in the proportion of CO2 that is locked in due to any of the first three mechanisms. The result is a mineralization of larger and larger amounts of CO2 that had hitherto been bound in any of the three preceding ways. Overall, these processes imply that storage security is increasing year by year.45

Seismic monitoring showing the diffusion of the injected quantity of CO2, its distribution and movement gives direct and clear indications of whether a suitable formation has been found. This means that injections into inappropriate formations can be cancelled at an early stage, and a number of measures can be taken upon indication of risk for leakage.

The Intergovernmental Panel on Climate Change (IPCC) has prepared a comprehensive report on carbon capture and storage.4 This report argues that it is “… likely that more than 99 % of stored CO2 stays in well-selected formations for more than 1000 years.” It is assumed that storage will persist during tens of millions of years, while probability statements beyond a thousand-year horizon are avoided.46

Figure 14 CO2 trapping in a saline aquifer. Diagram from the Special Report on Carbon Capture and Storage by the IPCC

3.4.3 Storage potential

The global capacity for storing CO2 is very large. According to IPCC estimates, there are storage facilities that can accommodate several trillion tonnes of CO2, see Table 1. For comparison, the annual global greenhouse gas emissions currently amount to some 30 billion tonnes of carbon dioxide.

Table 1. Storage capacity for different geological storage options in billion of tonnes of CO2.47

Reservoir type

Lower estimate of storage capacity in billion of tonnes of CO2(GtCO2)

Upper estimate of storage capacity in billion of tonnes of CO2(GtCO2)

Oil and gas fields

675

900

Unminable coal seams (ECBM)

3-15

200

Deep saline formations

1 000

10 000 (uncertain)

Total

1 675

˜11 000

Storage formations are relatively evenly spread out around the globe. There are excellent storage opportunities near many biomass facilities in North and South America, and proven storage opportunities exist in both Australia and Europe. Some regions of Africa and Asia are less well surveyed for CO2 storage locations, still it is believed that good storage opportunities exist also on these continents, which hold a large part of future BECCS opportunities.

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